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As more and more of the Earth’s surface is swallowed up by towns, roads
and agriculture, pressure grows on the remaining areas of truly wild habitat.
In the most developed parts of the world, only small pockets remain as sanctuaries
where wildlife can hang on. Growing concern over the dwindling number of
plant and animal species has led to a hunt for ways to save them, and in
the past decade, the idea of maintaining wildlife corridors – strips of
land to connect these sanctuaries – has become popular among conservationists.

It is an idea that is easy to grasp and has obvious, intuitive appeal.
Some of the most ambitious schemes cross national boundaries: for example,
there is a corridor linking the massive game reserves of Masai Mara in Kenya
and the Serengeti across the border in Tanzania. At the other end of the
scale are the modest efforts that make use of disused railway lines to
join patches of wasteland in inner cities, bringing a taste of the countryside
to town dwellers.

But it is as channels for the movement of rare or endangered species
that the idea has captured the imagination of environmentalists. The hope
is that wildlife corridors will allow plants and animals to colonise new
areas, or bring much-needed new blood into small populations that are in
danger through inbreeding. As a result, corridors are being hailed by some
conservation groups, especially in the US, as potential saviours of the
Florida panther, North American black bear (Ursus americanus) and many other
high-profile animals.

Hard sell

But not all scientists share this enthusiasm. There is a dearth of observational
evidence to show that corridors actually work, says Daniel Simberloff, professor
of biology at Florida State University, Tallahassee, who believes the concept
has been oversold. The huge costs of creating wildlife corridors may sometimes
outweigh the benefits, he says, and the money may be better spent setting
up and monitoring conventional reserves.

Most schemes that are in operation today are designed with individual
species such as tigers, elephants and other large mammals in mind. But advocates
of wildlife corridors say they could be used to preserve ecological communities.
Fears that rapid global warming might outpace species’ abilities to adapt
to changing conditions have led to proposals for corridors that would allow
animals and plants to migrate when conditions change. There is a plan on
the table for a 300-metre-wide ‘global change’ corridor stretching thousands
of kilometres through North America.

A corridor this wide linking Florida to Canada would cover some 720
square kilometres. Even if it were possible to buy this amount of land,
critics are sceptical about the likely results. Such a thin strip would
be useless for species which can only disperse slowly, says Simberloff.
‘If a corridor is intended to preserve an entire community, it is particularly
important that it is wide enough to permit breeding as well as movement.’

Simberloff believes that some corridors are being promoted for less
than creditable reasons, particularly in the forests of the northwestern
US. ‘I have dealt in various ways with enough logging companies and developers
to have seen a pattern,’ he says. ‘They offer skinny corridors of dubious
value, stating that conservation biology shows that these will preserve
biodiversity, then they seek to destroy much larger tracts.’

Thin end of the hedge

In 1985, for example, a construction company proposed a complex of condominiums
and golf courses for Fort George, a small island off the coast of north
Florida. The construction company hired an environmental consultant to produce
a report that looked at the best way to preserve the Antillian hard wood
habitat that naturally covered the island. The agency proposed leaving about
150 acres of the original habitat including corridors as thin as 10 metres.
Simberloff says this example is typical of many corridor proposals.

Biologists began investigating the theory of wildlife corridors in the
mid-1970s. The Harvard University zoologist Edward O. Wilson argued in his
book Ecology and Evolution of Communities, published in 1975, that corridors
were a protection against extinction. If a habitat has to be divided, ‘extinctions
will be lower when the fragments can be connected by corridors of natural
habitat, no matter how thin the corridors’, he wrote. The theory was later
developed using mathematical models of how communities of different species
interact (see ‘The case for corridors’). The idea of wildlife corridors
was adopted in the early 1980s and promoted by such influential international
conservation organisations as the World Wide Fund for Nature and the International
Union for the Conservation of Nature and Natural Resources (now renamed
the World Conservation Union). In December 1991, a report by the Institute
for European Environmental Policy, an EC-funded research organisation,
argued that ‘ecological corridors’ were an essential feature of a network
linking European nature reserves

Simberloff singles out the IEEP document as ‘a masterpiece of uncritical
thinking and glib generalisations’. Critics also point out that mathematical
models have yet to be tested in practice, and say that the evidence apparently
showing that mammals and birds will use corridors is flawed. Researchers
in the US and Australia did record plenty of sightings and trappings of
birds and small mammals in corridors during the 1980s. But Simberloff says
this evidence does not show whether the animals were using the strips of
land for migration or as a foraging area to supplement their home territory.

Simberloff is not the only scientist who is sceptical about the validity
of the evidence for the effectiveness of corridors. David Dawson of the
London Ecology Unit has just published a report on the subject for English
Nature, one of the British government’s main advisers on conservation.
He concludes that what firm evidence there is only shows movement in a
limited range of circumstances, between small, closely spaced patches in
an agricultural landscape.

One frequently quoted investigation is a study by Gray Merriam and colleagues
from Carleton University, Ottawa. Working in eastern Canada in the 1980s,
they deliberately removed fieldmice and chipmunks from patches of woodland,
then studied the animals’ movement along hedges that linked the patches.
The researchers found that the hedges helped these species to recolonise
the woods. ¿ìè¶ÌÊÓƵs do not, however, accept that this justifies most existing
corridors, which have been set up mainly to protect much larger, more glamorous
species such as bears and big cats. These animals may have home territories
stretching over several square miles. Nobody has yet proved that corridors
can work on such a scale, Dawson says.

Big ideas

Doing so would be a huge undertaking. Take the case of the Chila Motichur
corridor in India, between the Corbett and Rajaji National Parks in the
foothills of the Himalayas. Including adjacent areas of forest and small
sanctuaries, it covers an area of 2500 square kilometres. Indian researchers
have tried valiantly to assess movements of elephants, tigers and other
species along this enormous corridor, but the difficult terrain forces them
to rely on patchy evidence from dung samples.

Attempts to prove the value of wildlife corridors face conceptual problems,
too, their critics say. One is that the strips of land in question are anything
from a few metres to several kilometres wide. The Grider Creek corridor
that connects two forest wilderness areas in the extreme north of California
and southern Oregon is 5.5 kilometres wide and 26 kilometres long.

Such corridors are likely to function much better than narrower ones,
says Simberloff. For one thing, they provide real habitats, not just travel
routes. And in a strip there may be nowhere that is far enough away from
the edge to be insulated from the environment beyond the corridor. Weeds
that thrive in the surrounding farmland, for instance, may intrude into
the corridor and upset the ecological balance, while species such as woodland
birds may be vulnerable to attacks by predators such as hawks and crows.
At corridor edges, forest plants may also suffer from being exposed to too
much wind, light or humidity.

One of the justifications cited for wildlife corridors is the idea that
the genetic vitality of isolated populations is improved by the migration
of animals from outside. The standard argument is that matings between
close relatives result in unfavourable genetic characteristics being maintained,
when in a larger population they would be lost through genetic mixing.
But this picture is not always borne out by reality, says Simberloff. Species
such as the European bison (Bison bonasus) and the Pere David’s deer (Elaphurus
davidianus) have no such problems, even though they have been rescued from
the point of extinction and are now highly inbred. And African cheetahs
are extremely uniform genetically – possibly due to having come close to
extinction in the distant past – but do not appear to suffer for it. Small
populations do not necessarily become extinct, says Simberloff. Researchers
keeping tabs on the red-tailed hawk (Buteo jamaicensis socorroensis) on
the Pacific island of Socorro, off the coast of Mexico, have found that
the bird has survived for several decades as a stable population of about
twenty pairs. There is no reason to suppose the population was ever any
larger, they say.

If inbreeding did become a problem, one solution might be to trap a
few individuals from outside and release them into the isolated area, says
Simberloff. ‘The number of individuals that must be transferred among populations
can be surprisingly small – so small that it might be genetically advantageous
and far less expensive to move animals manually rather than to provide corridors.’
Transferring birds’ eggs between populations has been tried successfully
in many species, he says. And in the northern US, timber wolves (Canis
lupus) have been transported from one state to another.

Others are less impressed by this option. Reed Noss, a research associate
at Stanford University’s Center for Conservation Biology, questions the
ethics of ‘shipping animals about in crates’. He also warns that trapping
and transporting individuals would be impractical, except for a few species.
Critics of transportation also point out that it can never conserve entire
communities of plants and animals, only individual species.

Another pitfall, which could in principle apply to wildlife corridors
as well, is the problem of genetic dilution – the fear that mixing populations
could lead to the loss of useful local characteristics, a phenomenon known
as outbreeding depression. But Noss does not think that this is a real
drawback with wildlife corridors. ‘Maintaining or restoring natural landscape
connectivity has never been shown to cause outbreeding depression in wild
populations,’ he says.

Transmission route

Another worry is that corridors might become pathways for the transmission
of disease. The risk may be all the greater if the corridor connects small,
crowded populations of animals and plants, warns George Hess of North Carolina
State University. This effect can be seen where animals are crowded together
in a game reserve. Max Appel, a virologist at Cornell University veterinary
school in New York, has recently identified an outbreak of canine distemper
as having killed more than 60 lions in the Serengeti.

Wildlife corridors could also act as conduits for fire, predator species
or weeds that could disrupt the natural plant communities. That isolation
can benefit some species is evident from observations from New Zealand.
Wild pigs have wiped out Paryphanta snails from many of the larger forest
refuges, but they survive in small isolated patches that the pigs have not
been able to invade.

Simberloff cites conservation projects from his home state of Florida
as examples of ‘uncritical advocacy of extremely expensive corridors’. In
1990, the state’s Department of Natural Resources devised the Pal-Mar project
to create a 8860-hectare conservation area in Palm Beach and Martin counties.
The project was intended to preserve some of the best woodland and marsh
remaining in south Florida east of the Everglades. So far so good. But
later the department decided to extend the project to create a corridor
1.6 kilometres wide and 10 kilometres long that would connect Pal-Mar to
the Jonathan Dickinson State Park. Simberloff says that the proposal was
submitted without any accompanying biological data on which species would
use it, and why it was necessary. But more importantly, the proposed corridor
would cross two highways – the six lanes of Interstate 95 and the four-lane
Florida turnpike – both potentially lethal obstacles to animals trying to
use the route.

The department claimed that the corridor – which would have cost at
least $21 million – was the key to the whole Pal-Mar scheme and that the
necessary land should be bought before the reserve itself was acquired.
But this plan has now been put on the back burner, partly as a result of
fierce criticisms from Simberloff and like-minded critics. Work is going
ahead on the first stage of a three-phase plan to preserve the main wetland
system. The corridor phase may still go ahead, but only after the first
two stages have been completed.

Part of the problem stems from a shortage of money: earmark funds for
corridors, and there isn’t much left for anything else. Florida’s Conservation
and Recreation Lands programme had a budget of $185 million for acquiring
land in 1991. But 93 separate projects were given initial approval, and
the top 60 alone would have cost nearly $1 billion. Simberloff believes
that with resources so scare, every project ought to undergo a cost benefit
analysis – except that the information to show whether a corridor scheme
would be worthwhile is rarely if ever available.

Conservation biologists should be working towards making such an analysis
possible, says Dawson. Until it is, he says scientists should adopt the
precautionary principle and assume that existing corridors could be important.
Richard Hobbs, an ecologist based in Western Australia, sees corridors as
neither a cure-all nor a complete disaster. They ‘will benefit some species
but not others’, he says. ‘With careful planning they can form part of
a regional conservation network.

John Bonner is a freelance science writer.

* * *

The case for corridors

Conservationists who argued in the 1970s for the creation of wildlife
corridors based their case on the island theory of biodiversity. Observations
of the wildlife of island habitats have often shown that the number of species
depends on the size of the island and its distance from the mainland. Large
islands close to the mainland support more species than smaller more remote
ones. According to the theory, some species on a island will inevitably
become extinct through natural processes such as predation. However, individuals
of the same or different species will occasionally arrive to colonise the
island. The number of species will stabilise when the rate of extinctions
matches the rate at which replacements arrive: extinctions will be more
frequent on a small island and recolonisation more common over short distances.

These conservationists applied the same thinking to isolated patches
of original habitat, modelling them as islands in a hostile sea. A corridor
in effect draws the island closer to the mainland by increasing immigration,
raising the equilibrium number of species.

As more information has been gathered on the behaviour of isolated populations,
the island model has been replaced by a more complex mathematical theory.
This states that species consist of a set of different populations, known
as metapopulations, distributed over a number of patches that are connected,
to varying degrees, by immigration and emigration. The dynamics of a particular
metapopulation reflect changes within patches, and changes and dispersal
between patches. As with the island model it is assumed that corridors affect
metapopula-tion dynamics by increasing the ability of individuals to move
between patches.

Critics are sceptical about applying such mathematical theories to real
life. The results, they say, depend too much on the assumptions made in
constructing the model. The biggest concern is that the calculations do
not take into account problems such as the spread of disease, fire and genetic
dilution, which could undermine the good that corridors achieve.